Controversial children's book of the year Into The River will carry a warning that it is suitable for readers aged 16 years and older.

The Office of Film and Literature Classification this week delivered its findings on Ted Dawe's novel, saying people might be offended but the book would remain unrestricted.

It recommends, however, that the book be read by a mature audience.

Information and policy manager Kate Ward said there were already young-adult books with similar sexual descriptions, and to place a restriction on Dawe's novel would be unfair.

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Dawe welcomed the decision, saying it was based on a thorough and expansive analysis.

"It comes down squarely in support of the book's literary values," he said.

Into The River - laced with detailed descriptions of sex acts, coarse language and scenes of drug-taking - claimed top prize at this year's New Zealand Post Children's Book Awards.

In June, award organisers had hastily sent "explicit content" stickers to all booksellers after the book won the supreme award.

The classification decision is open for public review until October 9.